Control of continuous casting operation

ABSTRACT

A method and apparatus for continuously monitoring and controlling a continuous casting operation to detect and heal breakouts occurring periodically during the casting operation is disclosed. In a continuous casting operation wherein molten metal is progressively passed through three consecutive zones: a first zone wherein molten metal is passed from a holding vessel without significant solidification due to its low heat transfer capacity, a second zone wherein a thin skin of metal forms progressively due to its relatively high heat transfer capacity, and a third zone in which the molten metal solidifies to form a selfsustaining rod; a thermocouple is placed in the vicinity of the juncture of the first and second zones to measure the temperature fluctuation at the juncture. The advance of the solidified rod is automatically stopped on a continued decrease in temperature at the juncture as measured by the thermocouple which results from a breakout occurring in zone two. Stopping of the rod advance allows for the breakout to heat after which advance is automatically resumed.

llnited States Patent m1 Goodrich et al.

154] CONTROL OF CONTINUOUS CASTING OPERATION [75] Inventors: George M.Goodrich, Almont; Robert G. Williams, Birmingham; Fred J. Webbere,Orchard Lake, all of Mich.

[73] Assignee: General Motors Detroit, Mich.

22 Filed: July 7,1971

21 Appl.No.: 160,487

Corporation,

Related US. Application Data [63] Continuation-impart of Ser. No.879,832, Nov. 25,

1969, abandoned.

[451 Apr. M1, 1973 FOREIGN PATENTS OR APPLICATIONS 697,669 9/ 1953 GreatBritain ..164/82 1,087,026 10/ l 967 Great Britain 164/83 PrimaryExaminer-R. Spencer Annear Att0mey--Sidney Carter et al.

[57] ABSTRACT A method and apparatus for continuously monitoring andcontrolling a continuous casting operation to d etect and heal breakoutsoccurring periodically during the casting operation is disclosed. In acontinuous casting operation wherein molten metal is progressive- 1ypassed through three consecutive zones: a first zone wherein moltenmetal is passed from a holding vessel without significant solidificationdue to its low heat transfer capacity, a second zone wherein a thin skinof metal forms progressively due to its relatively high heat transfercapacity, and a third zone in which the molten metal solidifies to forma self-sustaining rod; a thermocouple is placed in the vicinity of thejuncture of the first and second zones to measure the temperaturefluctuation at the juncture. The advance of the solidified rod isautomatically stopped on a continued decrease in temperature at thejuncture as measured by the thermocouple which results from a breakoutoccurring in zone two. Stopping of the rod advance allows for thebreakout to heat after which advance is automatically resumed.

7 c, 9 Drawing Figures PATENTEDAPRIOIJYE 3,726,333

SHEET 1 [IF 2 ZONE 3 X2 ZONEZ +ZONE 1 A JAWS vln m W21 4 2 MS ZI M WWOWT Hgluz m MN], SM UG WM m s w n [m P LOW HEAT TRANSFER ZONE Z0 FLOW OFMOLTEN METAL i2 REVIOUSLY FORM MOLTEN METAL/ SEGMENT INVEWORS /ZZ @062WEAKEST POINT TEARS ADMITTING MOLTEN METAL TO MOLD SURFACE Di RECTION OFSKIN GROWTH PREVIOUSLY FORMED l' SEGMENT 9 FRESH LY FORMED SKIN FIRSTMETAL STAYS IN PLACE PATENIEDI-FR I 0W 3,726,333

SHEET 2 [IF 2 WELDgESILJRING I I I IIIIIIIIIIIIIIQ'IIIII I IgREVIOUSLYORMED FRESHLY FORMED SKN SEGMENT J? .6 T.C.

TEMP. TIMING 26R SENSOR cIRcuIT CONTROL EXTENDED DWELL TO NORMAL DWELLPERIOD CORRECT BREAKOUT HM {WWIHW NORMAL RESTORED NORMAL SOLIDIFICATIONSOLIDIFICATION TEMPERATURE V I BELOW WHICH SKIN RUPTURE/ FORWARD MOTIONOF RESTDUAL SENSING SYSTEM ED SKIN INDICATED BY IS ACTUATED TF. RISE INMOLD TEMP.

2 SECONDS T|ME (ZERO TIME) 900'F-' FIRST INDICATED RIsE IN MOLD TEMP.

I I I T MOLD 'COOLING :WATER ON INITIATIoN OF FIRST FORWARD STROKE Z v jE @vzye/ZZ boahc/z, l l I l l l 1 J l l l l l ME (fi m/J. Weere CONTROLOF CONTINUOUS CASTING OPERATION This application is acontinuation-in-part of our application Ser. No. 879,832 entitledControl of Continuous Casting Operation, filed Nov. 25, 1969 nowabandoned and assigned to the assignee of the present invention.

This invention relates to continuous casting of round bars or ingots,and more particularly to a method and apparatus for continuouslymonitoring and controlling the continuous casting operation.

This invention comprises an improvement over the method and apparatusfor continuous casting round bars and ingots, particularly rods having adiameter of about 1 to 3 inches, described in copending applications,Ser. No. 827,673 and Ser. No. 827,747, respectively, filed May 26, 1969,and assigned to the assignee of the present invention. That inventiondescribed, in general, a horizontally disposed open-ended moldassociated with a molten metal holding vessel or furnace including a lowheat transfer first zone immediately adjacent the holding vessel whichis effective to contain and convey molten metal therethrough withoutappreciable solidification and which is substantially chemically inertto the molten metal, a relatively high heat transfer second zoneadjacent the first zone which effects the initial solidification in theform of a thin skin of solidified metal progressively and coextensively,first at the interface of the first and second zones and progressivelyto the end of the second zone, and a third zone adjacent the second zonewherein the molten metal is further solidified to form a self-sustainingrod. The apparatus also includes means to mechanically pull thesolidified rod from the opposite end of the mold continuously butintermittently in predetermined increments and with predetermined timeintervals between the increments of length or segments of movement ofthe rod wherein each increment corresponds in length to the aforesaidsecond or initial solidification zone so that with each incremental pullof the rod the thin solidified skin layer or segment formed in thesecond zone is advanced into the third zone thereby exposing the secondzone to the advance of molten metal from the first zone and theprogressive solidification of a new skin layer or segment is formed inthe second zone. The time interval or dwell time between the pullingintervals is sufficient to permit the forward end of the newly formedskin layer in the second zone to weld to the rod in the third zone, sothat when the rod is pulled again, it will carry with it the newlyformed segment into the third zone in a continuous rod solidificationprocess.

It has been observed, however, that'periodically during the castingoperation breakouts can occur when the new skin layer or segment formedin the second zone fails to weld to the previously cast increment in thethird zone with sufficient strength to be pulled or stripped from themold wall during forward movement of the bar. Molten metal then issuesfrom the rupture in the solid skin and solidifies in an improper sectionof the mold resulting in disruptions in the normal cast bar surface. Ifbreakouts continue to occur during the casting operation, a series ofhumps or enlarged diameter increments on the bar surface are producedwhich make it difficult to pull the bar through the third zone and whichcause considerable scoring of the third zone mold wall. It has also beenfound that breakouts can be cured by a prolonged dwell in the castingcycle which allows for healing of the rupture. However, previous to theinvention hereinafter described it has been impossible to quickly detectwhen a breakout occurs and to stop the casting operation to allow forhealing of the rupture to take place.

Accordingly, it is among the principal objects of this invention toprovide apparatus for continuously monitoring the casting operationwhich will sense in a very short time when a breakout occurs and whichwill upon sensing a breakout automatically stop the casting operationfor an extending dwell period to allow for healing of the rupture.

It is another object of this invention to provide a method forcontinuously casting metal rods in which the solidified bar is advancedintermittently in predetermined increments with predetermined timeintervals between the increments and in which extended dwell periods areinserted in the intermittent advance of the rod when a breakout occursto allow for healing of the ruptured skin after which the intermittentadvance of the rod is again resumed.

It is a further object of this-invention to provide apparatus forcontinuously monitoring the casting operation in order to detectbreakouts and in order to obtain a reliable time reference for startingthe casting operation and to obtain a reliable indication of wear anderosion of the first zone of the mold during the casting operation.

These and other objects are accomplished by providing a thermocouplehaving its temperature sensing element in close proximity to thejuncture of the low heat transfer first zone and the relatively highheat transfer second zone and whose terminal leads are attached througha high speed temperature sensor and timing circuit apparatus to theapparatus for advancing the solidified bar. This invention is based on afinding that when a breakout occurs during the casting operation it isaccompanied by an improper formation of the skin layer at the junctureof the low heat transfer first zone and the relatively high heattransfer second zone which results in a change in the heat transfercharacteristics of the casting process which is indicated by a decreasein temperature at this juncture. By continuously monitoring thetemperature in the vicinity of this juncture by means of the sensingapparatus, the rod advancing means can be automatically topped to allowfor an extended dwell period in which the rupture heals after whichdwell period the rod advancing means automatically resumes intermittentadvance of the solidified rod and continuation of the casting operation.

Other objects and advantages of the invention will be apparent from thefollowing description, reference being had to the accompaning drawingsof which:

FIG. l is a cross-sectional view of a horizontal continuous castingapparatus;

FIG. 2 is an enlarged view of a portion of the mold shown in FIG. ll;

FIG. 3 to 6 are schematic illustrations of the mold at various stages ofthe casting process showing the solidification sequence in the mold;

FIG. 7 is a block diagram showing the arrangement of the temperaturemonitoring system;

FIG. 8 is a graph showing a mold temperature variation with castingcycles, skin rupture and corrective extended dwell, and

FIG. 9 is a graph showing the variation in mold temperature with startup.

Referring now to FIG. 1 in the drawings, the molding apparatus of thisinvention consists generally of a mo]- ten metal reservoir 10, shown asa fragment thereof, and a horizontally disposed open-ended mold 12mounted adjacent an opening 14 near the base of the reservoir. Thereservoir is of conventional construction including an outer metal shell(not shown) having a lining 16 of suitable refractory material forcontaining molten metal, such as steel. The opening or channel 14 in thereservoir is formed in a frusto-conical refractory body 18 cemented tothe lining 16. The refractory reservoir may include heating means suchas an induction heating coil or a resistance heating element formaintaining the metal at a desired temperature.

Referring to FIG. 2, the mold 12 consists of three distinctly differentportions with different heat transfer characteristics. The first portionimmediately adjacent the reservoir includes a nozzle portion 20preferably formed of boron nitride having relatively low heat transfercharacteristics such as will contain the molten metal therein withoutany appreciable solidification. The second portion 22 is positionedimmediately adjacent the nozzle 20 and is formed of a material having arelatively high heat transfer characteristic as, for example, aberyllium-copper alloy. The third portion 24 is disposed immediatelyadjacent the second portion 22 and preferably provided with a graphiteliner 26. The third portion preferably has somewhat lower heat transfercharacteristics than the second portion 22. The use of the graphiteliner is advantageous because it is relatively soft and self-lubricatingand it permits the solidified bar to be readily drawn through eventhough minor imperfections may have occurred on the surface of the rodduring solidification thereof. The design of the mold elements is suchthat there is a shoulder at the juncture 25 of the low heat transferzone and the relatively high heat transfer zone. The second mold portion22, as well as the third mold portion 24, are both provided with coolantpassages 28. The mold 12 is also provided with a thermocouple having itstemperature sensing element in close proximity to the juncture 25 of thelow heat transfer zone and the relatively high heat transfer zone andhaving its terminal ends extending outside of the mold.

After the process of this invention has been started and is incontinuous operation as hereinafter described, it is characterizedbasically by the molten metal passing from the reservoir 10 throughthree successive zones in the mold 12. The molten metal is conveyed fromthe reservoir 10 to the first zone without exposure to air whereby thebuildup of oxide deposits in the region of zone one is substantiallyprevented. No significant solidification occurs due to sufficiently lowheat transfer capacity of the first zone. As the metal flows into thesecond zone, a thin skin layer of solidified metal 32 is progressivelyformed along its length due to the high heat transfer'capacity of thesecond zone portion of the mold. This skin layer is then advanced as asegment or increment into the third zone of the mold wherein the moltenmetal is further solidified to form a self-sustaining rod which ismechanically pulled out of the mold by suitable means such as therollers 34. The rollers are clamped on the bar and ride on a slidingcarriage such that when the carriage is moved away from the mold, thebar is advanced; and when moved towards the mold in a direction oppositethat of the rollers, the bar remains stationary thereby providingintermittent advance of the rod. As the aforesaid skin'layer 32 isadvanced from the second zone to the third zone, a second skin layer isformed in the second zone which subsequently welds itself to the rodbeing solidified in the third zone. This skin layer is advanced into thethird zone as the rod is pulled incremently whereby a continuous rod isformed in a continuous but incremental process.

Periodically during the casting operation as previously described, it ispossible for breakouts to occur. The term breakout" refers to rupturesin the skin of the solidifying bar during its earliest stages offormation. If breakouts persist or will not heal, they may terminate thecasting operation. However, by monitoring the process previouslydescribed, the breakout problem can be cured.

The following detailed explanation will make the nature of the improvedprocess more clear, reference being had to FIGS. 2 to 6. The reservoir10 is provided with a suitable quantity of molten metal such as steel sothat its level extends substantially above the mold 12. The molten metaladvances due to gravity in the mold through the nozzle 20 whichconstitutes the aforementioned first zone. Since the first zone is madeof a material of relatively low heat conductivity and is not providedwith any cooling means, the molten metal does not significantly solidifytherein. As soon as the molten metal enters the second zone an initialcircumferential annulus solidifies against the mold surface portion 22,as shown in FIGS. 2 and 3. This occurs because the mold portion 22 isformed of a material of relatively high heat conductivity and is cooledby means of suitable coolants such as water circulating in the coolantpassages 28 to provide a high heat transfer capacity whereby a film orskin of metal 32 solidifies on the surface of the mold the instantcontact is made. As the molten metal advances into the second zone,solidified skin layer 32 forms progressively on the surface of the moldportion 22 in a downstream direction. This formation of skin layer 32occurs entirely within the mold portion 22 beginning at the juncture 25of zone one and zone two.

Referring to FIG. 4, the skin layer segment 32 is then advanced as asegment into a third zone of the mold wherein further solidificationtakes place to form the self-sustaining rod. As the skin layer 32 beginsto advance into the third zone, it must release from the nozzle 20 atthe interface 25 to form a slight space between the skin 32 and thenozzle. This space is immediately filled with molten metal flowing fromthe first zone to initiate the formation of a new skin layer at theinterface 25 with the nozzle and the mold portion 22 and to closelyfollow the advancing skin layer 32 and to progressively form a new skinlayer 36, as shown in FIG. 5. After the layer 32 has reached its fullincrement of movement, it is permitted to remain stationary for a timesufficient to permit the new layer 36 to weld to the layer 32. It isessential to the successful operation of the process that the skin layer36 part cleanly from the nozzle 20 and the skin layer 32 remainstationary for a time sufiicient to permit the new skin layer 36 to weldthereto. If either of these process steps are not performed properly, abreakout will occur in the successively formed skin layers causingmolten metal to breakout and to prevent proper rod solidification. Theuse of boron nitride for forming at least said portion of nozzleadjacent the mold portion 22 is highly advantageous because the skinlayer does not readily adhere to boron nitride and thereby results in aclean release. However, occasionally during the operation of theprocess, the freshly formed skin layer 36 does not part from the boronnitride nozzle at the juncture 25 on incremental movement of thepreviously formed segment 32 and a tear occurs at 38 admitting moltenmetal to the mold surface.

It is characteristic of this process that solidification occurs in adownstream direction beginning at the juncture of zone one and zone twoand that therefore, the weakest point is the juncture 38 of the freshlyformed skin 36 and the previously formed segment 32 which is the pointof last metal solidification and the point where the freshly formed skinis supposed to weld to the previously formed segment before incrementaladvance is resumed. Therefore, when a tear or breakout occurs at thisweak point, molten metal issues against this point of the mold resultingin an improper solidification sequence as compared to the normal castingprocess. Such a change in the solidification sequence alters the heattransfer characteristics in this region of the mold, i.e., zone two, andalso the heat transfer characteristic in zone one. It may be seen then,that the method for sensing breakouts depends essentially on sensing achange in the normal heat transfer characteristics of the castingprocess.

As the previously formed segment 32 is moved forward incrementally,molten metal continues to be admitted against the mold wall 22 in adownstream direction. As this molten metal is solidifying,solidification also continues at the juncture 25 of zone one and zonetwo thus increasing the thickness of the skin layer at the junction, asshown in FIG. 6. As this skin layer increases, it presents anincreasingly thick thermal barrier and as a consequence the temperatureat this junction as measured by the thermocouple 30 begins to decreasebelow the temperature of the normally molten metal entering andbeginning solidification at the junction during the normal operation ofthe process. This temperature decrease at this junction as measured bythe thermocouple is a practical method ofdetermining a change in heattransfer resulting from a breakout in the casting process.

in the casting process the solidified bar is being advanced inincrements of a fraction of a second, and therefore, it is mandatorythat a breakout be sensed very quickly. Rapid breakout detection hasbeen achieved through an automatic breakout sensing system, as shown inblock form in FIG. 7. The sensing unit utilizes a Chromel-Alumelthermocouple to detect the temperature in the vicinity of the junctureof zone one and zone two. The EMF from the thermocouple is amplified anddisplayed on a DC microammeter which has an adjustable low limitswitching capability. An emergency stoppage or extended dwell in thecasting operation is actuated when the juncture temperature decreasesbelow a preset lower limit on the microammeter by disengaging the rodpulling means from the rod.

Referring again to FIG. 6, it may be seen that during the extended dwellperiod the freshly formed skin 36 welds to the previously formed segment32. At the end of the preset extended dwell period, the rod advancingmeans is automatically actuated by the timing circuit to begin advanceof the rod again. If the breakout has been healed, the thermocouple willbegin to register an increase in temperature as molten metal begins toform a fresh skin at the junction 25 of zone one and zone two. However,if the dwell period has not been sufficient for the freshly formedsegment to weld to the previously formed segment the temperature willcontinue to decrease and the breakout sensing system will again beactuated through the temperature sensor, the timing circuit and the barpull control, as shown in FIG. 7, to disengage the rod advancing meansand the thereby provide for another extended dwell period for welding totake place between the freshly formed skin and the previously formedsegment. It has been found that two extended dwell periods are normallysufficient for complete healing of a breakout during the castingoperation.

The axial length of the skin layer or segment 32 has a practicallimitation and we have found that it should preferably be from 0.1 to1.5 times the diameter of the rod being cast. If the incremental pull orstroke is shorter, the casting rate is sacrificed and nozzle erosion isexcessive. If the stroke is longer, shrinkage porosity in the castingwill be excessive. By way of a specific example, a bar of about 1%inches in diameter is successfully cast with the segment 32 being aboutll inch in length. In casting stock of about 1% inches in diameter, wehave successfully used a segment of 1 inch in a cycle time of 0.25seconds, the cycle being the sum of the time consumed in drawing the barone segment as above described and the dwell time, i.e., the time therod is permitted to remain at rest. A typical dwell time is 0.12seconds. Satisfactory casting results are obtained with a variation ofdwell time from about 0.1 to 0.36 seconds with the proportion of thedwell to the cycle time being about 33 percent to 65 percent and withthe cycle time, accordingly, being about 0.15 to 1.1 seconds. We havefurther successfully used extended dwell periods of from 2 to 10 secondsfor complete healing of breakouts.

Referring to FIG. 8, the normal solidification process, breakout, andrestored normal solidification may be seen by comparing the incrementalmovement of the rod with the mold temperature at the juncture of zoneone and zone two. Reading from left to right, it may be seen that innormal operation the rod advances incrementally with a stroke of about 1inch with periodic dwell periods between strokes. During normaloperation the temperature fluctuates 10 30 F cyclicly with each strokefrom a maximum temperature shortly after the time when molten metalfirst enters the juncture of zone one and zone two on the advance of therod to a minimum temperature at the end of the dwell period whensolidification is completed in zone two and just prior to the advance ofthe rod. As seen at the center of the graph, a skin rupture or breakoutis sensed as the temperature continues to decrease below the normalminimum fluctuation temperature after a normal dwell period. When thetemperature decreases below this normal minimum temperature,'the sensingsystem is actuated and the rod advancing means stopped to allow for anextended dwell period to correct the breakout. In practical applicationswe have found that excellent results can be obtained when the sensingsystem is actuated when the temperature falls below a fixedpredetermined temperature, for example to 45 F below the minimumtemperature, as shown in FIG. 8. During the extended dwell, thetemperature continues to decrease as solidification proceeds in zone twoand the thickness of the skin layer at the juncture between zone one andzone two increases. At the end of the predetermined fixed extended dwellperiod forward motion of the solidified rod is again resumed and forwardmotion of the residual or previously unwelded skin is indicated by arise in mold temperature as fresh molten metal enters zone two.Observing the right side of the graph, it may be seen that after theextended dwell, the normal solidification mode is restored and thetemperature fluctuations again follow the normal rise and fall withincremental motion.

As the skin layer moves into zone three, a progressively greater radialthickness of the molten metal solidifies therein as the bar advances toeventually form a self-sustaining bar which is pulled mechanically fromthe zone three, as shown in FIG. 2.

It will be apparent to one skilled in the art that numerous variationsof the monitoring system herein described can be made. As previouslydescribed, the thermocouple, located preferably in the vicinity of thejuncture of zone one and zone two, registers a decrease in moldtemperature at the juncture when a breakout occurs. This decrease inmold temperature is a result of a change in the normal heat transfercharacteristics of the molding process due to an improper mode ofsolidification in zone two and is-reflected progressively in adownstream direction at all points along zone two, beginning at thejuncture of zone one and zone two, by a decreasing temperature. It maybe seen then, that the thermocouple may be placed at any point alongzone two and will measure a temperature decrease corresponding to thatat the juncture with equal sensitivity but at a slightly later timebecause the skin is solidifying into an increasingly thick thermalbarrier in a downstream direction away from the juncture. In addition,zone one will also reflect a temperature decrease but with lesssensitivity at points remote from the juncture because of the relativelylow heat transfer capacity of the zone one material. Thus, since theeffect actually being monitored is a change in heat transfercharacteristics in zone two which is also reflected in zone one, thethermocouple or any suitable temperature sensitive device will beoperative in the molding and monitoring process hereinabove described ifit is positioned so as to sense changes in mold temperature in a regionbetween the beginning of zone one and the beginning of zone three.

It will be further apparent to one skilled in the art that more than onethermocouple may be used in the monitoring process. That is, when onethermocouple is used in the monitoring process the change in moldtemperature is referenced with respect to room temperature. Since themold temperature changes progressively in a downstream direction, twothermocouples may be positioned along the mold in the region between thebeginning of the first zone and the beginning of the third zone with thechange in mold temperature being referenced to the differentialtemperature between the thermocouples. The practical effect of thismethod is the decrease in the amount of fluctuation in temperature, aspreviously seen in FIG. 8, and thereby lend increased sensitivity to thetemperature monitoring process. The sensing system is actuated for anextended dwell period on the occurrence of a breakout when thedifferential temperature between the thermocouples falls below theminimum differential in temperature normally occurring in operation or,alternatively, below a predetermined differential in temperature.

The method of this invention has particular utility in casting metalswhich have a relatively high melting temperature of about 2200 of ormore, and are unsaturated in carbon so that they cannot be cast orsolidified in a graphite mold because of their tendency to absorb carbonby diffusion. Of particular importance in this class of metals areferrous metals such as steel, and other ferrous metals typicallycontaining up to about 2 percent carbon. Illustrative of metal which maybe cast in accordance with this invention are SAE 4118 steel containing,by weight, 0.18 percent to 0.23 percent carbon, 07 percent to 0.9percent manganese, 0.4 percent to 0.6 percent chromium, 0.08 percent to0.15 percent molybdenum, 0.04 percent maximum, phosphorus, 0.04 percentmaximum, sulphur, and the balance essentially iron; SAE 5160 steelcontaining, by weight, 0.55 percent to 0.65 percent carbon, 0.75 percentto 1.0 percent manganese, 0.2 percent to 0.9 percent chromium, 0.04percent maximum, phosphorus, 0.04 percent maximum, sulphur, and thebalance essentially iron; SAE 52100 steel containing, by weight, 0.95percent to 1.1 percent carbon, 0.25 percent to 0.45 percent manganese,1.3 percent to 1.6 percent chromium, 0.25 percent maximum, phosphorus,0.25 percent maximum, sulphur, and the balance essentially iron. Nickelbased alloys and cobalt based alloys containing predominant amounts ofnickel or cobalt may also be successfully cast in accordance with theprocess of this invention.

Illustrative of a nickel base alloy of this type is Inconel 610consisting of 0.2 percent carbon, 1.0 percent manganese, 9 percent iron,1.6 percent silicon, 0.5 percent copper, 15.5 percent chromium,columbium plus tantalum about 2 percent, and the balance essentiallynickel; and Rene 41 consisting of about 18 percent to 20 percentchromium, 10 percent to 12 percent cobalt, 9 percent to 10.5 percentmolybdenum, 5 percent iron, 0.09 percent to 0.12 percent carbon, 0.5percent silicon, 0.1 percent manganese, 3 percent to 3.3 percenttitanium, 1.4 percent to 1.6 percent aluminum, and the balance nickel.Illustrative of a cobalt based alloy which may be cast in accordancewith the invention is Haynes 25, consisting of 0.05 percent to 0.15percent carbon, 1.9 percent to 2 percent manganese, 19 percent to 21percent chromium, 9 percent to 11 percent nickel. 14 percent to 16percent tungsten, 3 percent iron, 1 percent silicon, and the balanceessentially cobalt.

As described above, the metal solidification occurs in a metal moldportion of the zone two so that there is no carbon source for carbondiffusion. The skin. layer 32 is well formed when it is transferred tothe graphite lined zone three so that no appreciable carbon diffusionoccurs during the casting process.

The above description of the process and apparatus is applicable tonormal operational conditions after the molding process has commenced.The process is commenced by inserting a suitable rod (not shown) intothe exit end of the mold until it reaches approximately to the junctureof the first and second zones. The initial molten metal flowing into themold is permitted to flow against the rod end and to bond thereto. Thisrod is then pulled incrementally as described above to establish thecasting process. This same temperature sensing arrangement as previouslydescribed can also be used to advantage in obtaining a reliable timereference for starting the casting operation. Start-up is a particularlycritical operation in any continuous casting operation because of itssensitivity to metal temperature, preheat of the casting vessel, moldcondition, presence of entrapped slag, and gas and other variables. Ithas been found that an initial extended delay is required after thefirst metal enters the mold to ensure complete solidification of thecast material around the anchoring projection at the end of the starterbar. The length of this delay will vary with the size of the bar and themetal composition. Referring to FIG. 9, it may be seen that the rise inthe mold temperature indicated by the mold thermocouple located near thejuncture of zone one and two gives an accurate readout of entry of thefirst metal and can be used as a time reference for either manual orautomatic initiation of the casting process in which initiation of thecasting process is delayed for an extended dwell period after whichinitiation of the first forward stroke is begun. A continued rise inmold temperature indicates that the solidification process is proceedingnormally with each forward stroke. At a given time after casting hasbegun, the temperature fluctuations within the mold will establish astable pattern as previously seen in FIG. 8.

It has also been observed that the temperature sensing apparatus isoperative to determine the amount of nozzle wear and erosion at thejuncture of zone one and zone two. Since the length of time that castingcan be continued is depended on the integrity of the juncture betweenzone one and zone two, the control apparatus adds another controldimension to the casting operation. As the process proceeds, thethermocouple located at the juncture of zone one and zone two registersa gradual increase in temperature during extended casting periods. Thisgradual increase in temperature is indicative to wear and erosion of thematerial which forms the low heat transfer zone that overlaps the highheat transfer material. The magnitude of the gradual increase can,therefore, be utilized to determine when the casting process should beterminated because of extreme nozzle erosion conditions and therebyprevent permanent mold damage.

This invention has been described with reference to horizontalcontinuous casting only, however, those skilled in the art willrecognize that it may be easily adapted to vertical continuous castingthrough such obvious modifications as placing the opening 14 in thebottom of the reservoir and aligning the mold vertically with saidrevervoir. In the vertical position the method embodying this inventionmay be used to control the vertical casting operation.

Although this invention has been described in tenns of specificexamples, it is to be understood that other forms of the invention maybe readily adaptedwithin the scope of the invention.

We claim:

1. A method of continuously casting a continuous rod comprising thesteps of:

a. continuously introducing molten metal into the inlet end of thecavity of an open-ended continuous casting mold; said mold including afirst zone adjacent said inlet end having a relatively low heat transfercapacity, a second zone immediately adjacent said first zone having arelatively high heat transfer capacity, and a third zone adjacent saidsecond zone terminating in an open outlet end;

' c. advancing said molten metal into said first zone, the heat transfercharacteristics of said first zone being operative to substantiallyprevent any metal solidification therein;

d. advancing said molten metal into said second zone, the high heattransfer characteristics of said second zone being operative to causesolidification of a thin layer of said metal at first immediately at thejuncture of said first and second zone and then progressive- -ly in thedirection of said third zone whereby a thin layer of said molten metalis formed coextensively of said second zone;

e. advancing said rod intermittently in fixed predetermined incrementsat fixed predetermined time intervals,- said increments being equal tothe length of said second zone whereby said thin layer of saidsolidified metal formed in said second zone is advanced progressivelyinto said third zone and the molten metal advancing from said first zonefollows the advance of said thin layer to progressively fill said secondzone immediately behind said advancing thin layer and to therebyprogressively form a new thin solidified layer coextensively of saidsecond-zone cavity surface, said intervals being of sufficient durationto permit the forward axial end of said newly formed thin layer to weldto the solidified metal in said third zone;

f. continuously monitoring the change in mold temperature in a regionbetween the beginning of said first zone and the beginning of said thirdzone, said mold temperature normally fluctuating between a maximumtemperature and a minimum temperature during said time interval;

stopping the advance of said rod for a fixed predetermined dwell periodwhen said mold temperature falls below said minimum temperature whichindicates that said newly formed thin layer has failed to weld to thesolidified metal in said 1 third zone, said dwell period being longerthan said fixed time intervals, to permit said layer, which previouslyfailed to weld, to weld to the solidified metal in said third zone, and

tion of said dwell period in said fixed predeter? .mined increments atsaid fixed predetermined time intervals. I

2. A method of continuously casting a continuous rod comprising thesteps of:

a. continuously introducing molten metal into the inlet end of thecavity of an open-ended continuous casting mold;

b. said mold including a first zone adjacent'said inlet end havingarelatively low heat transfer capacity, a second zone immediatelyadjacent said first zone resuming the advance of said rod at thecomplehaving a relatively high heat transfer capacity, and a third zoneadjacent said second zone terminating in an open outlet end;

c. advancing said molten metal into said first zone, the heat transfercharacteristics of said first zone being operative to substantiallyprevent any metal solidification therein;

d. advancing said molten metal into said second zone, the high heattransfer characteristics of said second zone being operative to causesolidification of a thin layer of said metal at first immediately at thejuncture of said first and second zones and then progressively in thedirection of said third zone whereby a thin layer of said molten metalis formed coextensively of said second zone;

e. advancing said rod intermittently in fixed predetermined incrementsat fixed predetermined time intervals, said increments being equal tothe length of said second zone whereby said thin layer of saidsolidified metal formed in said second zone is advanced progressivelyinto said third zone and the molten metal advancing from said firstzonefollows the advance of said thin layer to progressively fill saidsecond zone immediately behind said advancing thin layer and to therebyprogressively form a new thin solidified layer coextensively of saidsecond zone cavity surface, said intervals being of sufficient durationto permit the forward axial end of said newly formed thin layer to weldto the solidified metal in said third zone;

f. continuously monitoring the mold temperature in the vicinity of saidjuncture, said mold temperature normally fluctuating between a maximumtemperature and a minimum temperature during said time interval;

g. stopping the advance of said rod for a fixed predetermined dwellperiod when said mold temperature falls below a predeterminedtemperature, said predetermined temperature being below said minimumtemperature, which indicates that said newly formed thin layer hasfailed to weld to the solidified metal in said third zone, said dwellperiod being longer than said fixed time intervals, to permit saidlayer, which previously failed to weld, to weld to the solidified metalin said third zone, and

h. resuming the advance of said rod at the completion of said dwellperiod in said fixed predetermined increments at said fixedpredetermined time intervals.

3. The method of claim 2 wherein said predetermined temperature is about15 to 45 F below said minimum temperature.

4. A method of continuously casting a continuous rod comprising thesteps of:

a. continuously introducing molten metal into the inlet end of thecavity of an open-ended continuous casting mold;

b. said mold including a first zone adjacent said inlet end having arelatively low heat transfer capacity, a second zone immediatelyadjacent said first zone having a relatively high heat transfercapacity, and a third zone adjacent said second zone terminating in anopen outlet end;

c. advancing said molten metal into said first zone, the heat transfercharacteristics of said first zone being operative to substantiallyprevent any metal solidification therein;

d. advancing said molten metal into said second zone, the high heattransfer characteristics of said second zone being operative to causesolidification of a thin layer of said metal at first immediately at thejuncture of said first and second zone and then progressively in thedirection of said third zone whereby a thin layer of said molten metalis formed coextensively of said second zone; and

e. advancing said rod continuously but in fixed increments of 0.1 to 1.5times the diameter of the rod being cast at fixed cycle time intervalsof about 0.15 to 1.1 seconds, said increments being equal to the lengthof said second zone whereby said thin layer of said solidified metalformed in said second zone is advanced progressively into said thirdzone and the molten metal advancing from said first zone follows theadvance of said thin layer to progressively fill said second zoneimmediately behind said advancing thin layer and to therebyprogressively form a new thin solidified layer coextensively of saidsecond zone cavity surface, said rod being at rest for a time of 33percent to percent of said cycle time interval to permit the forwardaxial end of said newly formed thin layer to weld to thesolidified metalin said third zone;

f. continuously monitoring the change in mold temperature in a regionbetween the beginning of said first zone and the beginning of said thirdzone, said mold temperature normally fluctuating between a maximumtemperature and a minimum temperature during said time interval;

g. stopping the advance of said rod for a fixed predetermined dwellperiod of about 2 to 10 seconds when said mold temperature falls belowsaid minimum temperature which indicates that said newly formed thinlayer has failed to weld to the solidified metal in said third zone topermit said layer, which previously failed to weld, to weld to thesolidified metal in said third zone, and

h. resuming the advance of said rod at the completion of said dwellperiod in said fixed predetermined increments at said fixedpredetermined time intervals.

5. The method of claim 4 wherein said metal is a ferrous metalcontaining less than 2percent by weight carbon.

6. Apparatus for continuous casting of a metal ingot comprising, incombination, an open-ended mold having an inlet end and an outlet endand a molten metal reservoir associated with said inlet end in sealedfluid flow relationship,

said mold including a first portion, including said inlet end having arelatively low heat transfer capacity disposed adjacent said reservoir,a second portion adjacent said first portion having a relatively highheat transfer capacity, and a third portion adjacent said secondportion,

- the heat transfer capacities of said first portion and said secondportion being related so that molten metal flowing through said mold ismaintained in a substantially completely molten state within said firstmold portion and the high heat transfer capacity of said second portionis operative to form at least a skin layer of solidified metal on thesurface thereof beginning immediately at the juncture of said firstportion and said second portion,

temperature sensing means located between the beginning of said firstportion and the beginning of said third portion to sense normal moldtemperature fluctuations occurring during casting,

means for advancing said ingot formed in said mold intermittently inpredetermined increments and at predetermined time intervals, and

means responsive to said'temperature fluctuations to stop said advancingmeans for a fixed predetermined dwell period when said mold temperaturefalls below the minimum temperature of said fluc-' tuations and torestart said advancing means at the completion of said dwell period.

7. Apparatus for continuous casting of metal ingot said mold including afirst portion, including said inlet end having a relatively low heattransfer capacity disposed adjacent said reservoir, a second portionadjacent said first portion having a relatively high heat transfercapacity, and a third portion adjacent said second portion,

the heat transfer capacities of said first portion and said secondportion being related so that molten metal flowing through said mold ismaintained in a substantially completely molten state within said firstmold portion and the high heat transfer capacity of said second portionis operative to form at least a skin layer of solidified metal on thesurface thereof beginning immediately at the juncture of said firstportion and said second portion,

a thermocouple having its sensing element in close,

proximity to said juncture to sense normal mold temperature fluctuationsoccurring during casting,

1. A method of continuously casting a continuous rod comprising thesteps of: a. continuously introducing molten metal into the inlet end ofthe cavity of an open-ended continuous casting mold; b. said moldincluding a first zone adjacent said inlet end having a relatively lowheat transfer capacity, a second zone immediately adjacent said firstzone having a relatively high heat transfer capacity, and a third zoneadjacent said second zone terminating in an open outlet end; c.advancing said molten metal into said first zone, the heat transfercharacteristics of said first zone being operative to substantiallyprevent any metal solidification therein; d. advancing said molten metalinto said second zone, the high heat transfer characteristics of saidsecond zone being operative to cause solidification of a thin layer ofsaid metal at first immediately at the juncture of said first and secondzone and then progressively in the direction of said third zone wherebya thin layer of said molten metal is formed coextensively of said secondzone; e. advancing said rod intermittently in fixed predeterminedincrements at fixed predetermined time intervals, said increments beingequal to the length of said second zone whereby said thin layer of saidsolidified metal formed in said second zone is advanced progressivelyinto said third zone and the molten metal advancing from said first zonefollows the advance of said thin layer to progressively fill said secondzone immediately behind said advancing thin layer and to therebyprogressively form a new thin solidified layer coextensively of saidsecond zone cavity surface, said intervals being of sufficient durationto permit the forward axial end of said newly formed thin layer to weldto the solidified metal in said third zone; f. continuously monitoringthe change in mold temperature in a region between the beginning of saidfirst zone and the beginning of said third zone, said mold temperaturenormally fluctuating between a maximum temperature and a minimumtemperature during said time interval; g. stopping the advance of saidrod for a fixed predetermined dwell period when said mold temperaturefalls below said minimum temperature which indicates that said newlyformed thin layer has failed to weld to the solidified metal in saidthird zone, said dwell period being longer than said fixed timeintervals, to permit said layer, which previously failed to weld, toweld to the solidified metal in said third zone, and h. resuming theadvance of said rod at the completion of said dwell period in said fixedpredetermined increments at said fixed predetermined time intervals. 2.A method of coNtinuously casting a continuous rod comprising the stepsof: a. continuously introducing molten metal into the inlet end of thecavity of an open-ended continuous casting mold; b. said mold includinga first zone adjacent said inlet end having a relatively low heattransfer capacity, a second zone immediately adjacent said first zonehaving a relatively high heat transfer capacity, and a third zoneadjacent said second zone terminating in an open outlet end; c.advancing said molten metal into said first zone, the heat transfercharacteristics of said first zone being operative to substantiallyprevent any metal solidification therein; d. advancing said molten metalinto said second zone, the high heat transfer characteristics of saidsecond zone being operative to cause solidification of a thin layer ofsaid metal at first immediately at the juncture of said first and secondzones and then progressively in the direction of said third zone wherebya thin layer of said molten metal is formed coextensively of said secondzone; e. advancing said rod intermittently in fixed predeterminedincrements at fixed predetermined time intervals, said increments beingequal to the length of said second zone whereby said thin layer of saidsolidified metal formed in said second zone is advanced progressivelyinto said third zone and the molten metal advancing from said first zonefollows the advance of said thin layer to progressively fill said secondzone immediately behind said advancing thin layer and to therebyprogressively form a new thin solidified layer coextensively of saidsecond zone cavity surface, said intervals being of sufficient durationto permit the forward axial end of said newly formed thin layer to weldto the solidified metal in said third zone; f. continuously monitoringthe mold temperature in the vicinity of said juncture, said moldtemperature normally fluctuating between a maximum temperature and aminimum temperature during said time interval; g. stopping the advanceof said rod for a fixed predetermined dwell period when said moldtemperature falls below a predetermined temperature, said predeterminedtemperature being below said minimum temperature, which indicates thatsaid newly formed thin layer has failed to weld to the solidified metalin said third zone, said dwell period being longer than said fixed timeintervals, to permit said layer, which previously failed to weld, toweld to the solidified metal in said third zone, and h. resuming theadvance of said rod at the completion of said dwell period in said fixedpredetermined increments at said fixed predetermined time intervals. 3.The method of claim 2 wherein said predetermined temperature is about15* to 45* F below said minimum temperature.
 4. A method of continuouslycasting a continuous rod comprising the steps of: a. continuouslyintroducing molten metal into the inlet end of the cavity of anopen-ended continuous casting mold; b. said mold including a first zoneadjacent said inlet end having a relatively low heat transfer capacity,a second zone immediately adjacent said first zone having a relativelyhigh heat transfer capacity, and a third zone adjacent said second zoneterminating in an open outlet end; c. advancing said molten metal intosaid first zone, the heat transfer characteristics of said first zonebeing operative to substantially prevent any metal solidificationtherein; d. advancing said molten metal into said second zone, the highheat transfer characteristics of said second zone being operative tocause solidification of a thin layer of said metal at first immediatelyat the juncture of said first and second zone and then progressively inthe direction of said third zone whereby a thin layer of said moltenmetal is formed coextensively of said second zone; and e. advancing saidrod continuously but in fixed increments of 0.1 to 1.5 times thediameter of the rod being cast aT fixed cycle time intervals of about0.15 to 1.1 seconds, said increments being equal to the length of saidsecond zone whereby said thin layer of said solidified metal formed insaid second zone is advanced progressively into said third zone and themolten metal advancing from said first zone follows the advance of saidthin layer to progressively fill said second zone immediately behindsaid advancing thin layer and to thereby progressively form a new thinsolidified layer coextensively of said second zone cavity surface, saidrod being at rest for a time of 33 percent to 65 percent of said cycletime interval to permit the forward axial end of said newly formed thinlayer to weld to the solidified metal in said third zone; f.continuously monitoring the change in mold temperature in a regionbetween the beginning of said first zone and the beginning of said thirdzone, said mold temperature normally fluctuating between a maximumtemperature and a minimum temperature during said time interval; g.stopping the advance of said rod for a fixed predetermined dwell periodof about 2 to 10 seconds when said mold temperature falls below saidminimum temperature which indicates that said newly formed thin layerhas failed to weld to the solidified metal in said third zone to permitsaid layer, which previously failed to weld, to weld to the solidifiedmetal in said third zone, and h. resuming the advance of said rod at thecompletion of said dwell period in said fixed predetermined incrementsat said fixed predetermined time intervals.
 5. The method of claim 4wherein said metal is a ferrous metal containing less than 2percent byweight carbon.
 6. Apparatus for continuous casting of a metal ingotcomprising, in combination, an open-ended mold having an inlet end andan outlet end and a molten metal reservoir associated with said inletend in sealed fluid flow relationship, said mold including a firstportion, including said inlet end having a relatively low heat transfercapacity disposed adjacent said reservoir, a second portion adjacentsaid first portion having a relatively high heat transfer capacity, anda third portion adjacent said second portion, the heat transfercapacities of said first portion and said second portion being relatedso that molten metal flowing through said mold is maintained in asubstantially completely molten state within said first mold portion andthe high heat transfer capacity of said second portion is operative toform at least a skin layer of solidified metal on the surface thereofbeginning immediately at the juncture of said first portion and saidsecond portion, temperature sensing means located between the beginningof said first portion and the beginning of said third portion to sensenormal mold temperature fluctuations occurring during casting, means foradvancing said ingot formed in said mold intermittently in predeterminedincrements and at predetermined time intervals, and means responsive tosaid temperature fluctuations to stop said advancing means for a fixedpredetermined dwell period when said mold temperature falls below theminimum temperature of said fluctuations and to restart said advancingmeans at the completion of said dwell period.
 7. Apparatus forcontinuous casting of metal ingot comprising, in combination, anopen-ended mold having an inlet end and an outlet end and a molten metalreservoir associated with said inlet end in sealed fluid flowrelationship, said mold including a first portion, including said inletend having a relatively low heat transfer capacity disposed adjacentsaid reservoir, a second portion adjacent said first portion having arelatively high heat transfer capacity, and a third portion adjacentsaid second portion, the heat transfer capacities of said first portionand said second portion being related so that molten metal flowingthrough said mold is maintained in a substantially completely moltenstate within said first mold portion and the high heat transfer capacityof said second portion is operative to form at least a skin layer ofsolidified metal on the surface thereof beginning immediately at thejuncture of said first portion and said second portion, a thermocouplehaving its sensing element in close proximity to said juncture to sensenormal mold temperature fluctuations occurring during casting, means foradvancing said ingot formed in said mold intermittently in predeterminedincrements and at predetermined time intervals, and means responsive tosaid temperature fluctuations to stop said advancing means for a fixedpredetermined dwell period when the mold temperature at said juncturefalls below a predetermined temperature, said predetermined temperaturebeing below the minimum temperature of said fluctuations, and to restartsaid advancing means at the completion of said dwell period.